In a conventional electrical power conversion apparatus for an electric railroad car, a converter is connected to a single-phase AC power source between a power line and a rail/rails as the AC-side input by way of a pantograph and a transformer, or the like, and performs an AC-to-DC conversion so as to obtain a predetermined DC voltage. A capacitor is provided on the DC side of the converter for smoothing its voltage. DC power stored in the capacitor is converted into variable frequency and/or variable voltage AC power by the inverter. An AC rotating machine such as an induction motor is driven by the AC power that the inverter outputs. In addition, the voltage of the capacitor, namely, a DC input voltage for the inverter is detected by a voltage detector. On the AC output side of the inverter, a current detector is provided.
The voltage of the capacitor pulsates at a two-fold frequency (the frequency is referred to as a “ripple frequency”) of a frequency of the AC power source. It is known that, if no countermeasure is taken in a case in which a frequency (referred to as an “output frequency”) of AC power that an inverter outputs is near to the ripple frequency, the AC power that the inverter outputs and, consequently, torque produced by a motor vary at a frequency of the difference between the ripple frequency and the output frequency. Such variation is referred to as a “beat phenomenon.” The beat phenomenon occurs when a difference in power arises between a positive half-cycle and a negative half-cycle of the AC power because of the DC voltage that varies.
In order to eliminate or curb the beat phenomenon, a control is performed in which only a ripple component is extracted using a band-pass filter or the like from a DC input voltage for an inverter detected by a voltage detector, and, in one period of a ripple frequency, an output frequency of the inverter is varied in proportion to the magnitude of a ripple component. (For example, refer to Patent Document 1.)
[Patent Document 1]    Japanese Patent No. H07-46918 (FIG. 1, claim 1)